1. Field of the Invention
The present invention relates generally to a process for determining a remaining life of a component suffering from creep due to operation of the component (or, part) under relatively high centrifugal or thermal loads, and more specifically to a process for determining a remaining life of an airfoil such as a rotor blade or a stator vane suffering from creep due to operation under relatively high centrifugal or thermal loads from operation in a gas turbine engine.
2. Description of the Related Art Including Information Disclosed Under 37 CFR 1.97 and 1.98
High temperature and stress are common operating conditions for various parts in a gas turbine engine, such as rotor blades and stator vanes in the compressor or the turbine. Creep and stress rupture are some principal types of elevated temperature mechanical failure modes. Generally, the type of failure is established by examination of fracture surfaces and comparison of component operating conditions with available data on creep, stress rupture, tension, elevated temperature fatigue and thermal fatigue properties.
Creep deformation can produce large changes in the dimensions of a part to either render the part useless for further service or cause fracture. In an airfoil used in a gas turbine engine, excessive creep can cause the airfoil to untwist—which would reduce the aerodynamic efficiency of the airfoil—or can cause the airfoil (if it is a rotor blade) to rub excessively against the blade outer air seal (BOAS). Creep is also responsive for causing grain boundary cavitations, which leads to micro cracks that eventually lead to full size cracks that can cause fracture. Thus, excessive creep deformation causes the material to reach or exceed some design limit on strain, which is referred to as creep failure. In a gas turbine engine, creep failure can be the bowing of a stator vane due to excessive thermal and pressure loads occurring on the vane or creep crack in a turbine vane.
Creep deformation can also lead to fracture of an airfoil. Fracture can occur from either localized creep damage or more widespread bulk damage caused by the accumulation of creep strains over time. Structural components that suffer from bulk creep damage typically are exposed to uniform loading and uniform temperature distribution during the operating life. This type of failure is referred to as stress rupture or creep rupture.
Failures from creep deformation depend on the alloy, the time-temperature exposure, loading conditions, component geometry, and also environmental and metallurgical factors. Corrosion, fatigue or material defects can also contribute to creep and stress-rupture defects. Creep deformation becomes important when mechanical strength of a metal becomes limited by creep rather than by yield strength. This transition in engineering design is not directly related to melting temperature. The temperature at which the mechanical strength of a metal becomes limited by creep, rather than by elastic limit, must be determined individually for each metal or alloy.
Constant load bulk deformation creep curves typically consist of three distinct stages as seen in FIGS. 1 and 2. FIG. 1 is a strain curve for the three stages of creep under constant-load testing and constant-stress testing. FIG. 2 shows the relationship of strain rate, creep rate, and time during a constant-load creep test. The minimum creep rate is attained during second-stage creep. The first stage is called primary creep and is the region of the initial instantaneous elastic strain from the applied load. The region of secondary creep is where the creep rate is nominally constant at a minimum rate and is generally known as the minimum creep rate. The third and last stage is the region of tertiary creep, where drastically increased strain rate with rapid extension to fracture occurs.
In a gas turbine engine, such as an industrial gas turbine (IGT) or an aero gas turbine engine, airfoils such as rotor blades and stator vanes in the turbine or compressor sections are exposed to extremely high centrifugal or thermal loads during engine operation. In the turbine section, the rotor blades are exposed to relatively high thermal and centrifugal loads as well as pressure loads. As mentioned above, when an engine part, such as a rotor blade in the turbine, suffers from excessive creep, the blade can untwist to the point where the aerodynamic efficiency of the blade is decreased, the blade can rub excessively against the blade outer air seal, or the blade can fracture due to crack growth. In either case, it would be beneficial for the engine operator to know if a certain engine part such as the rotor blade still has any remaining useful life when the engine is shut down and the parts are inspected for damage. Especially in an industrial gas turbine engine, which operates for long periods of time between shutdowns, reusing a damaged part will cause loss of efficiency or even engine damage due to part failure.